142 Mr. H. A. Rowland on Magnetic Permeability, 



riation of k; but he plots it with reference to It as abscissa 

 instead of R k, and thus fails to determine the law. His method 

 of experiment is much more complicated than mine, so that he 

 could only obtain results for one ring ; w r hile by my method 

 I have experimented on about a dozen rings and on numerous 

 bars, so that I believe I have been enabled to find the true form 

 of the function according to which fju varies with the magnetism 

 of the bar or the magnetizing-force. 



Many experiments have been made on the magnetism of iron 

 without giving the results in absolute measure. Among these 

 are the experiments of M tiller, Joule, Lenz and Jacobi, Dub, and 

 others. The experiments have been made by the attraction of 

 electromagnets, by the deflection of a compass-needle, or, in 

 one case, by measuring the induced current in a helix extend- 

 ing the whole length of the bar. By the last two methods the 

 change in the distribution of magnetism over the bar when the 

 magnetism of the bar varies is disregarded, if indeed it was 

 thought of at all : even in a recent memoir of M. Cazin* we 

 have the statement made that the position of the poles is inde- 

 pendent of the strength of the current. He does not give the 

 experiment from which he deduces this result. Now it is very 

 easy to show, from the formula of Green for the distribution of 

 magnetism on a bar-magnet combined with the known variation 

 of K t that this can only be true for short and thick bars ; and it 

 has also been remarked by Thomson that this should be the 

 casef. An experiment made in 1870 places this beyond doubt. 

 A small iron wire (No. 16), 8 inches long, was wound with two 

 layers of fine insulated wire ; a small hard steel magnet \ inch 

 long suspended by a fibre of silk was rendered entirely astatic 

 by a large magnet placed about 2 feet distant ; the wire electro- 

 magnet was then placed near it, so that the needle hung \\ inch 

 from it and about 2 inches back from the end. On now exciting 

 the magnet with a weak current, the needle took up a certain 

 definite position, indicating the direction of the line of force at 

 that point. When the current was very much increased, the 

 needle instantly moved into a position more nearly parallel to 

 the magnet, thus showing that the magnetism was now distri- 

 buted more nearly at the ends than before. This shows that 

 nearly all the experiments hitherto made on bar-magnets contain 

 an error ; but, owing to its small amount, we can accept the re- 

 sults as approximately true. 



I believe mine are the first experiments hitherto made on this 

 subject in which the results are expressed and the reasoning 

 carried out in the language of Faraday's theory of lines of 



* Annates de Chimie et de Physique, Feb. 1873, p. 171. 

 t Papers on Electricity and Magnetism, p. 512. 



